Microwave-assisted synthesis of ZnS@CuInxSy for photocatalytic degradation of coloured and non-coloured pollutants

Copper indium sulfide (CuInS2) exhibits strong visible light absorption and thus has the potential for good photocatalytic activity; however, rapid charge recombination limits its practical usage. An intriguing strategy to overcome this issue is to couple CuInS2 with another semiconductor to form a heterojunction, which can improve the charge carrier separation and, hence, enhance the photocatalytic activity. In this study, photocatalysts comprising CuInS2 with a secondary CuS phase (termed CuInxSy) and CuInxSy loaded with ZnS (termed ZnS@CuInxSy) were synthesized via a microwave-assisted method. Structural and morphological characterization revealed that the ZnS@CuInxSy photocatalyst comprised tetragonal CuInS2 containing a secondary phase of hexagonal CuS, coupled with hexagonal ZnS. The effective band gap energy of CuInxSy was widened from 2.23 to 2.71 as the ZnS loading increased from 0 to 30%. The coupling of CuInxSy with ZnS leads to long-lived charge carriers and efficient visible-light harvesting properties, which in turn lead to a remarkably high activity for the photocatalytic degradation of brilliant green (95.6% in 5 h) and conversion of 4-nitrophenol to 4-nitrophenolate ions (95.4% in 5 h). The active species involved in these photocatalytic processes were evaluated using suitable trapping agents. Based on the obtained results, photocatalytic mechanisms are proposed that emphasize the importance of h+, O2•–, and OH− in photocatalytic processes using ZnS@CuInxSy.


Experimental section Chemicals
All reagents were used without further purification.For the synthesis, copper nitrate trihydrate (Cu(NO 3 ) 2 •3H 2 O) and indium nitrate (In(NO 3 ) 3 ) were purchased from Alfa Aesar as the Cu and In sources, respectively.Zinc acetate dihydrate (Zn(CH 3 COO) 2 •2H 2 O), thiourea (CH 4 N 2 S), hydrogen peroxide (H 2 O 2 ), and 4-NP (C 6 H 5 NO 3 ) were purchased from Merck, Germany.The distilled water was purified using water still from Aquatron, England, and ethanol was purchased from Duksan Pure Chemicals Co. Ltd, South Korea.For the photocatalytic application tests, BG (C 27 H 34 N 2 O 4 S) was obtained from Sigma-Aldrich.For the trapping experiments, isopropanol and benzoquinone were obtained from Acros, and ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA) was purchased from Fluka.

Instrumentation
The CuIn x S y and ZnS@CuIn x S y composites were synthesized using an Anton-Paar microwave reactor (Monowave 400, Austria).The structure and phase purity of the synthesized materials were analyzed using powder X-ray diffraction (XRD, MiniflexII; Rigaku, Japan).X-ray photoelectron spectroscopy (XPS) was performed using a Kratos Analytical, AXIS Nova to determine the chemical states and elemental compositions of the synthesized materials.Fourier transform infrared spectroscopy (FT-IR) was used to identify the vibrational modes present in the synthesized materials.The FT-IR spectra of the synthesized materials were recorded using an IRspirit Fourier transform infrared spectrometer (IRSpirit, Shimadzu, Japan) in the wavenumber range 400-4000 cm −1 using the KBr pellet method.Raman spectra were obtained using a JASCO NRS-5100 Micro Raman Spectrometer equipped with a 532.06 nm laser.The optical band gap energies of the materials were determined using ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS, Shimadzu UV-2600i, Japan).Further morphological and crystallographic information about the synthesized materials was obtained using field-emission transmission electron microscopy (FE-TEM) and selected area electron diffraction (SAED) conducted with a JEM-F200 (JEOL Ltd., Tokyo, Japan).The average pore size, pore volume, and BET surface area of CuIn x S y and 30% ZnS@ CuIn x S y were measured using a surface area analyzer (Quantachrome autosorb-iQ, Austria).For photocatalytic studies of the synthesized materials, the experiments were carried out in a Toption (TOPT-V) photochemical reactor with a 300 W Xe lamp as the UV-visible light source (wavelength > 350 nm).The intensity of the light at the location of the reaction vessel was ~ 14 mW/cm 2 , as measured by a Thorlabs PM100D power meter with a S401C thermal sensor.A Shimadzu UV-1900 UV-visible spectrophotometer was utilized to monitor the absorbance of BG and 4-NP in the spectral range 200-800 nm.A general-purpose pH meter (GP353 EDT direction, United Kingdom) was used to determine the pH of the 4-NP aqueous solution.

Synthesis of CuIn x S y
CuIn x S y was synthesized using a simple microwave-assisted synthesis method as reported in a previous study 28 .In a typical synthesis, 0.2416 g of Cu(NO 3 ) 2 •3H 2 O, 0.3008 g of In(NO 3 ) 3 , and 0.3045 g of thiourea were added into 15 mL ethylene glycol and the mixture was loaded into a 30 mL quartz vessel (G30 vial).The vessel was purged with N 2 gas and sealed with a septum then rapidly heated to 200 °C by 850 W microwave irradiation for 10 min with continuous stirring to ensure homogenous heating.After the resulting product was cooled to ambient temperature, it was washed three times with distilled water and ethanol and collected by centrifugation (3500 rpm, 5 min per wash).Finally, the product was dried at 80 °C for 4 h to yield a black powder.Although pure, stoichiometric CuInS 2 was the intended synthesis product, the formation of a CuS secondary phase is almost inevitable; therefore, we use the term CuIn x S y throughout the manuscript to simplify the notation.

Synthesis of ZnS@CuIn x S y composites
For the synthesis of ZnS@CuIn x S y composites containing different weight percentages (%) of ZnS, the procedure is similar to that mentioned above.In brief, a certain amount of Zn(CH 3 COO) 2 •H 2 O and thiourea were added into a suspension of 0.2 g of CuIn x S y .The mixtures were purged with N 2 gas and heated to 200 °C by 850 W microwave irradiation for 10 min with continuous stirring.The obtained products were washed three times with distilled water and ethanol, dried at 80 °C for 4 h, and labelled as 10% ZnS@CuIn x S y , 20% ZnS@CuIn x S y , and 30% ZnS@CuIn x S y .

Photocatalytic degradation of brilliant green dye
The photocatalytic activity of the as-prepared materials was evaluated by degrading BG dye at room temperature under visible light irradiation.All experiments were carried out in triplicate.A typical process was as follows: 10 mg of synthesized material was dispersed into 50 mL of 10 ppm BG aqueous solution.Then, the suspension was sonicated for 3 min, and it was later kept in the dark for another 3 min with constant stirring to reach an adsorption-desorption equilibrium of BG dye on the surface of the synthesized materials.The suspension was then irradiated with visible light and 3 mL aliquots were collected at intervals of 60 min for 5 h and transferred into centrifuge tubes to separate the photocatalyst from the dye pollutant solution.The aliquots were analyzed using a UV-Vis spectrophotometer in the range 200-800 nm and the photocatalytic activities of the pristine CuIn x S y , 10% ZnS@CuIn x S y , 20% ZnS@CuIn x S y , and 30% ZnS@CuIn x S y were estimated by measuring the percentage of dye degradation using the following relation: where A 0 denotes the initial absorbance (the absorbance at time t = 0 h) and A t denotes the absorbance of the aqueous dye solution after time t of treatment.Both A 0 and A t are recorded at the absorbance maximum (λ max ) of the aqueous BG dye at 620 nm.

Photocatalytic conversion of 4-NP
For the photocatalytic conversion of 4-NP, 10 mg of synthesized material was added and sonicated in 50 mL of 10 ppm 4-NP solution.After 3 min of adsorption/desorption equilibrium, the photocatalytic 4-NP conversion was initiated by irradiating the reaction mixture with a 300 W Xenon lamp.The activity was monitored using a UV-vis spectrophotometer.At fixed intervals of 1 h, 3 mL of aliquots were collected and filtered by centrifugation to separate the photocatalyst before analysis of the solution.

Active species trapping experiments
To further investigate the main reactive species responsible for the photocatalytic degradation of BG using 30% ZnS@CuIn x S y composite, trapping experiments were carried out in the presence of three typical trapping agents: isopropanol, benzoquinone, and disodium ethylenediaminetetraacetate (EDTA-2Na), which are utilized as scavengers of • OH, O 2 •-, and h + , respectively.For 4-NP conversion, H 2 O 2 was used to trap e − in addition to other trapping agents.These trapping agents were added to the aqueous BG dye solution/4-NP solution at the beginning of the photocatalytic reaction.This experiment was carried out under the same conditions used for the BG degradation and 4-NP conversion experiments.

Results and discussions Powder X-ray diffraction analysis
Powder XRD was used to examine the crystal structures and phases of CuIn x S y , ZnS, and ZnS@CuIn x S y composites with different percentages of ZnS (10%, 20%, and 30%), and the results are displayed in Fig. 1.The XRD pattern of CuIn x S y indicated the formation of the tetragonal phase of CuInS 2 (ICDD 98-060-0582: indexed in blue) along with the hexagonal phase of CuS (ICDD 98-006-7581: indexed in orange), as shown in Figure S1.This phenomenon suggests that Cu 2+ ions could not be completely reduced by ethylene glycol 23 .The diffraction peaks of ZnS are well-matched with the reported data (ICDD 98-004-2793).The characteristic peaks located at 28.8°, 48.0°, 51.9°, and 56.5° can be observed clearly and corresponds to (002), ( 110), (103), and (112) of hexagonal ZnS (as indexed in green), respectively 38 .All of the synthesized ZnS@CuIn x S y composites exhibited the main diffraction peaks of CuInS 2 , CuS, and ZnS.The successful loading of ZnS was confirmed by observation of the broad ZnS diffraction peaks around 28° and 48° superimposed on the CuInS 2 and CuS diffraction patterns in the composites, as illustrated in Figure S2.Thus, XRD results confirmed the successful synthesis of ZnS@CuIn x S y using microwave-assisted synthesis using ethylene glycol as the solvent.

X-ray photoelectron spectroscopy
The chemical composition and oxidation states of the elements present in CuIn x S y and 30% ZnS@CuIn x S y were examined by XPS, as shown in Fig. 2. As depicted in the survey XPS (Fig. 2a), peaks corresponding to Cu 2p, In 3d, S 2p, and Zn 2p are observed clearly confirming the successful formation of CuIn x S y and ZnS@CuIn x S y composites.Figure 2b shows Cu 2p doublets at Cu 2p 3/2 (931.8/929.7 eV) and Cu 2p 1/2 (951.9/949.6 eV) for CuIn x S y and 30% ZnS@CuIn x S y , respectively.The results are consistent with the presence of Cu(I), as expected for stoichiometric CuInS 2 , and in agreement with previous reports 32,39 .Moreover, the Cu 2p spectra showed no evidence of Cu 2+ (in the form of Cu 2+ 'shake-up' satellites around 944 and 965 eV) 40 on the surface of both CuIn x S y and 30% ZnS@CuIn x S y , which indicates that the CuS secondary phase observed in the XRD patterns is not present on the surface of the photocatalysts.The In 3d XPS spectrum in Fig. 2c exhibited two peaks at 444.1 eV, 451.7 eV for CuIn x S y and 442.4 eV, 450.0 eV for 30% ZnS@CuIn x S y which correspond to In 3d 5/2 and In 3d 3/2 , respectively.This suggests that the oxidation state of In in the synthesized materials is + 3, as expected for CuInS 2

29
. Figure 2d shows that the binding energies of S 2p in CuIn x S y are at 162.7 and 161.7 belonging to S 2p 1/2 and S 2p 3/2 , respectively and were separated by a spin-orbit splitting of 1.2 eV.Moreover, the peak positions of S 2p for 30% ZnS@CuIn x S y are located at 160.4 and 159.2 eV 41 .The Zn 2p peaks of ZnS@CuIn x S y as shown in Fig. 2e split into Zn 2p 3/2 (1019.4eV) and Zn 2p 1/2 (1042.6 eV) can be assigned to Zn 2+ with a peak separation of 23.2 eV 41 .The typical C 1s spectra shown in Fig. 2f arise from adventitious carbon.The XPS results also verify the successful synthesis of ZnS@CuIn x S y composite via a microwave-assisted synthesis.

Fourier transform infrared spectroscopy
The vibrational modes present in the synthesized materials were determined via FT-IR.As shown in Fig. 3a, the band located at ~ 500-530 cm −1 can be assigned to the Cu-S 42 and In-S 43 stretching vibrations.Moreover, no other peaks can be observed in the FT-IR spectra, which reveals that all organic molecules from ethylene glycol and possible byproducts of the synthesis reaction were removed by washing with ethanol and distilled water 44,45 .For pure ZnS, the bands around 462 cm −1 and 667 cm −1 can be attributed to the characteristic Zn-S stretching vibration modes 46,47 .While, the bands around 1620 and 2070 cm −1 can be ascribed to the C-N and C=S vibrations of thiourea 48 .The FT-IR spectra of 10% ZnS@CuIn x S y , 20% ZnS@CuIn x S y , and 30% ZnS@CuIn x S y exhibited similar Cu-S, In-S, and Zn-S bands.This confirms the presence of both CuIn x S y and ZnS.Thus, FTIR measurements also verify the successful loading of ZnS onto CuIn x S y .www.nature.com/scientificreports/

Raman spectroscopy
Raman spectroscopy was employed to study the structure and bonding in CuIn x S y , ZnS, 10% ZnS@CuIn x S y , 20% ZnS@CuIn x S y, and 30% ZnS@CuIn x S y .Figure 3b shows the Raman scattering spectra of CuIn x S y and ZnS@ CuIn x S y composites with different ZnS contents.The Raman features broaden and shift continuously upward upon increasing ZnS content from 10 to 30%.The Raman spectrum of CuIn x S y exhibited a strong peak at 290 cm −1 which may be assigned to the A 1 mode of the chalcopyrite CuInS 2 49 .A weak peak at 470 cm −1 may be assigned to the S-S stretching vibration mode of hexagonal CuS 50,51 .Moreover, no detectable peaks were observed for ZnS.Thus, the Raman spectra can only confirm the presence of CuInS 2 and a secondary phase of CuS, which is in agreement with the XRD results.

UV-visible diffuse reflectance spectroscopy
The effective band gap energies of the synthesized materials were determined by constructing Tauc plots from Kubelka-Munk transformed diffuse reflectance data, as shown in Fig. 4. The inset of Fig. 4 shows the photographic of synthesized CuIn x S y , ZnS, 10% ZnS@CuIn x S y , 20% ZnS@CuIn x S y , and 30% ZnS@CuIn x S y .The Kubelka-Munk function was used to estimate the effective band gap energy.The estimated band gap energy of CuIn x S y was found to be 2.23 eV.While, for pure ZnS, the Tauc plot reveals a band gap energy of 3.87 eV, which corresponds to the sharp absorption onset at ~ 323 nm.As the ZnS content increases, the band gap energy of CuIn x S y gradually increases from 2.23 to 2.51 eV.The effective band gaps of 10% ZnS@CuIn x S y , 20% ZnS@ CuIn x S y , and 30% ZnS@CuIn x S y were estimated to be 2.32, 2.41, and 2.51 eV respectively.The ZnS@CuIn x S y composites exhibit narrow band gaps, which enable the materials to harvest visible light efficiently.

Transmission electron microscopy
The TEM images of CuIn x S y and 30% ZnS@CuIn x S y are shown in Fig. 5a and b, respectively.Figure 5a 1 and a 2 shows the rod-like structure of CuIn x S y particles.While 30% ZnS@CuIn x S y exhibited a needle-like structure, as shown in Fig. 5b 1 and b 2 , which also shows that ZnS was well dispersed on the surface of the CuIn x S y particles, and their close contact resulted in the formation of a ZnS@CuIn x S y composite, which is helpful for fast interfacial charge carrier transfer.Thus, the photogenerated charge carriers can be utilized effectively, thereby enhancing the photocatalytic performance.Figure 6a 1 shows the HR-TEM image of CuIn x S y , the spacing of the lattice fringe of 0.276 and 0.319 nm matched well with (020) and (112) planes of tetragonal CuInS 2 , respectively.The 0.319 nm lattice fringe was also observed in Fig. 6b 1 , the HR-TEM image of ZnS@CuIn x S y composites.The additional lattice fringe of 0.192 nm was assigned to the (110) plane of hexagonal ZnS.The SAED patterns of CuIn x S y in Fig. 6a 2 confirmed the presence of the ( 112) and (024) planes of tetragonal CuInS 2 (indexed in blue) and the (023) plane of hexagonal CuS (indexed in orange).Moreover, the SAED patterns of 30% ZnS@CuIn x S y as shown in Fig. 6b 2 exhibit bright concentric rings corresponding to the (112) and (224) diffraction planes of tetragonal CuInS 2 in addition to the (110) plane of hexagonal ZnS (indexed in green).These results are in accordance with the XRD results, which also confirm the successful loading of ZnS onto CuIn x S y .

Brunauer-Emmett-Teller surface area analysis
The BET N 2 adsorption-desorption isotherm analysis was carried out to investigate the average pore size, pore volume, and BET surface area of CuIn x S y and 30% ZnS@CuIn x S y .As shown in Fig. 7, the N 2 adsorption/desorption isotherm of CuIn x S y and 30% ZnS@CuIn x S y revealed a type IV isotherm (according to the IUPAC classification) indicating the mesoporous feature of the materials.From Table 1, the CuIn x S y exhibited a surface area of 46.9 m 2 /g, and with the introduction of 30% ZnS onto CuIn x S y , the surface area increased to 55.8 m 2 /g.The BET analysis revealed that the 30% ZnS@CuIn x S y has a surface area with pore size and pore volume of approximately 142.7 Å and 0.398 cm 3 /g, respectively.This suggests that the addition of ZnS could provide more active surface sites.Thus, providing enough active sites for the adsorption of dyes on the surface of the photocatalyst and photocatalysis to take place.

Photocatalytic degradation of brilliant green
The photocatalytic efficiencies of CuIn x S y , 10% ZnS@CuIn x S y , 20% ZnS@CuIn x S y, and 30% ZnS@CuIn x S y for BG dye are presented in Fig. 8a.Prior to light irradiation, the dye and photocatalyst suspension were kept in the dark for 3 min under constant stirring to achieve an adsorption-desorption equilibrium.The percentages of BG adsorption were found to be 41.5% ± 3.96%, 53.7% ± 2.93%, 51.4% ± 1.05%, and 60.7% ± 2.06% for CuIn x S y , 10% ZnS@CuIn x S y , 20% ZnS@CuIn x S y, and 30% ZnS@CuIn x S y , respectively, after 5 h in the dark, as shown in Figure S3.This shows that the CuIn x S y and ZnS@CuIn x S y composites have a high adsorptive affinity towards BG dye, which is crucial because if the dye molecules cannot be adsorbed on the surface of the synthesized materials, the photocatalytic activity would not be effective 52 .This may also be associated with the larger surface area, pore volume, and pore size of 30% ZnS@CuIn x S y compared with CuIn x S y , as shown in Table 1.During the course of the photocatalytic reaction, the intensity of the BG dye solution gradually diminishes, and a gradual decrease in the absorption spectra of BG can be observed at the characteristic absorption peak height around 620 nm.Moreover, unmodified CuIn x S y exhibited a poor photocatalytic performance of about 44.5% ± 1.36% under irradiation by visible light, which may be ascribed to the rapid recombination of charge carriers.The photocatalytic performance of CuIn x S y is significantly enhanced as the amount of ZnS loaded increases from 10 to 30%.The photocatalytic activity of 30% ZnS@CuIn x S y against BG was found to be considerably higher than those of unmodified CuIn x S y , 10% ZnS@CuIn x S y , and 20% ZnS@CuIn x S y .Within 5 h, 10% ZnS@CuIn x S y , 20% ZnS@ CuIn x S y, and 30% ZnS@CuIn x S y were able to degrade about 79.2% ± 1.66%, 92.0% ± 0.50%, and 95.6% ± 0.08% of BG, respectively under visible light irradiation.The improved photocatalytic performance of 30% ZnS@CuIn x S y is likely due to the presence of ZnS on the surface of CuIn x S y , which facilitates the separation and transfer of the photogenerated electrons and holes effectively resulting in superior degradation efficiency.Three different scavengers including benzoquinone for O 2 •-, EDTA for h + , and isopropanol for • OH were used to evaluate the main active species involved in the photocatalytic degradation of BG.As shown in Fig. 8b, the photocatalytic degradation of BG hardly decreased with the addition of isopropanol in comparison to other scavengers.Moreover, when benzoquinone was used to scavenge O 2 •-, a notable decrease in photocatalytic efficiency of 30% ZnS@CuIn x S y .The addition of EDTA showed the highest inhibition of the photocatalytic degradation of 30% ZnS@CuIn x S y which confirmed the influence of h + in the photocatalytic process.

Photocatalytic conversion of 4-NP
The photocatalytic conversion of 4-NP to the 4-nitrophenolate ion by CuIn x S y , 10% ZnS@CuIn x S y , 20% ZnS@ CuIn x S y, and 30% ZnS@CuIn x S y , as evidenced by UV-Vis absorbance spectroscopy, is shown in Fig. 9.In both the absence and presence of light, CuIn x S y exhibited poor photocatalytic conversion activity in comparison to Table 1.Average pore size, pore volume, and BET surface area of CuIn x S y and 30% ZnS@CuIn x S y .

Synthesized material
Average pore size (Å) Average pore volume (cm 3 /g) Surface area (m 2 /g)  the other synthesized materials, as shown in Fig. 9a and b.Interestingly, when the ZnS@CuIn x S y composites were added and irradiated with visible light, the maximum absorption peak of 4-NP shifted from 316 to 400 nm, and the colour of the solution changed from colourless to pale yellow due to the deprotonation of 4-NP to form the 4-nitrophenolate ion 53 .The conversion of 4-NP was gradually improved as the amount of ZnS loaded on the surface of CuIn x S y increased from 10 to 30%.Among the synthesized materials, 30% ZnS@CuIn x S y showed the highest conversion under irradiation by visible light.The active species involved in the process of 4-NP conversion were further identified by adding BQ, EDTA, isopropanol, and H 2 O 2 to the solution mixture as scavengers to capture O 2 •-, h + , • OH, and e − , respectively.Based on the reactive species trapping experiments (Fig. 9c), the addition of benzoquinone, isopropanol, and H 2 O 2 resulted in equal inhibition of 4-NP conversion, which indicates that O 2 •-, • OH, and e − play equal roles in the conversion process.Moreover, the addition of EDTA to the mixture exhibited the highest inhibition of 4-NP conversion, which implies that photogenerated h + are the main active species involved in the conversion process.

Proposed photocatalytic mechanism
The proposed photocatalytic mechanism of BG degradation using ZnS@CuIn x S y is illustrated in Fig. 10 based on the band gap energy, the reported energies (specified as potentials vs. NHE) of the conduction band (CB) and valence band (VB) of ZnS and CuInS 2 , and the trapping experiments.The secondary CuS phase is not separately considered here because the XPS results indicate that it is not present at the surface of the photocatalyst particles.Based on the literature, CuIn x S y is assumed to have E CB = − 0.34 V and E VB = 1.23 V 14 , while ZnS has E CB = − 1.56 V and E VB = 3.06 V 54 .Upon visible light irradiation, ZnS and CuIn x S y simultaneously generate e − and h + , in which photogenerated e − are excited to their CB, leaving behind h + in the VB.Next, the photogenerated e − in the CB of ZnS are transferred to the VB of CuIn x S y , which improves the separation of photogenerated charge carriers and prolongs their lifetime.This also increases the probability of the photogenerated charge carriers participating in Figure 9. Photocatalytic conversion of 4-NP using CuIn x S y , 10% ZnS@CuIn x S y , 20% ZnS@CuIn x S y, and 30% ZnS@CuIn x S y (a) in the dark, (b) under visible light irradiation, and (c) photocatalytic activity of 30% ZnS@ CuIn x S y with different trapping agents to determine the main reactive species responsible for the photocatalytic conversion of 4-NP., − 0.33 V vs. NHE), which agrees with the results of the active species trapping experiments.Alternatively, the photogenerated e − can reduce the BG dye directly.Moreover, the photogenerated h + of ZnS can react with H 2 O or OH − species, oxidizing them into • OH radicals, or the h + can directly oxidize the BG dye to CO 2 and H 2 O, which are harmless end products.Based on the active species trapping study, EDTA showed the highest inhibition followed by benzoquinone when compared to no scavenger, which indicates that h + and O 2 •-play important roles in the photocatalytic degradation process.Moreover, isopropanol shows the lowest inhibition, implying that • OH is the least important species in the photocatalytic degradation of BG by ZnS@CuIn x S y .
In the case of the photocatalytic conversion of 4-NP, the enhanced conversion may be ascribed to the presence of OH − , which can either be formed through (i) direct reduction of H 2 O by e − cb , (ii) reduction of O 2 to O 2 •-and subsequent reaction with H 2 O to form H 2 O 2 and OH − , or (iii) a two-step process in which H 2 O is first oxidized to • OH by h + vb and then e − cb reduces • OH to yield OH − ions.It must be noted that in order for these reactions to result in a net increase in pH, some h + must be consumed in a process other than water oxidation and/or H + must be consumed by another process (e.g., conversion of 4-NP and/or 4-nitrophenolate ion to hydroquinone, as evidenced by the peak located ~ 225 nm in Fig. 9) 55 .The presence of OH − increases the pH of the solution from 7.00 to slightly basic ~ 7.22, which deprotonates 4-NP to the 4-nitrophenolate ion.It is worth noting that the significance of the quite small pH change is dependent on the pK a of 4-NP.Since the pK a of 4-NP is 7.15 56 , this pH change is expected to cause a shift in the acid-base equilibrium from most of the 4-NP being protonated ([A -]/ [HA] ≈ 0.71, according to the Henderson-Hasselbalch equation) to most being deprotonated ([A -]/[HA] ≈ 1.17), which is consistent with the observed changes in absorbance 57 .

Conclusion
A series of highly efficient ZnS@CuIn x S y nanocomposite photocatalysts with different ZnS loadings have been successfully synthesized via a microwave-assisted method and applied for the photocatalytic degradation of BG dye and 4-NP in aqueous solution.When compared to unmodified CuIn x S y , the composite materials showed enhanced photocatalytic performance under visible light irradiation, particularly 30% ZnS@CuIn x S y .Active species trapping experiments indicate that mainly h + and O 2 •-are involved in the photocatalytic processes.The formation of a heterojunction between CuIn x S y and ZnS decreases photogenerated charge carrier recombination and thereby enhances the carrier separation efficiency in the composite.Therefore, this study provides a simple and effective route for the synthesis of a new visible-light active photocatalyst material and also highlights the importance of its application in the elimination of different organic contaminants from wastewater.

Figure 5 .
Figure 5. TEM images of (a 1 and a 2 ) CuIn x S y and (b 1 and b 2 ) 30% ZnS@CuIn x S y at two different magnifications.

Figure 6 .
Figure 6.HR-TEM images and SAED patterns of (a 1 and a 2 ) CuIn x S y , and (b 1 and b 2 ) 30% ZnS@CuIn x S y respectively.The Miller indices given in blue, orange, and green indicate the diffraction planes arising from CuInS 2 , Cus, and ZnS, respectively.

Figure 7 .
Figure 7. N 2 adsorption and desorption isotherms of CuIn x S y and 30% ZnS@CuIn x S y .

Figure 8 .
Figure 8. (a)Percentage photocatalytic degradation of BG using CuIn x S y , 10% ZnS@CuIn x S y , 20% ZnS@ CuIn x S y, and 30% ZnS@CuIn x S y under visible light irradiation and (b) photocatalytic activity of 30% ZnS@ CuIn x S y with different trapping agents to determine the main reactive species responsible for the photocatalytic degradation of BG.

Figure 10 .
Figure 10.Proposed mechanism for the photocatalytic degradation of BG and conversion of 4-NP using ZnS@ CuIn x S y .